1. Field of the Invention.
This invention relates to polyphase motors, and more particularly to a clutch connected between the rotor and the output shafts of the motor for allowing the output shaft to slip thereby preventing the transmission of excessive torque to the load upon start up of the motor.
2. Description of the Prior Art.
Polyphase induction motors, particularly three phase motors, are very common because of their simple construction, efficiency, good speed regulation and low cost. Such motors come in all sizes and find wide use where polyphase power is available. Both squirrel-cage and wound rotor machines are in common use because both can be designed for two phase or three phase current, the wound rotor type providing both high starting torque and good speed regulation and being the most common type for high power applications.
An inherent problem with such motors is their start up. Some motors of this type reach their maximum speed in a second, and the torque applied by the power output shaft an the downstream components is very high, at times 600% of the rated torque of the motor As a result, the downstream components are usually built heavier than necessary, and thus are more costly, in order to absorb the start up `jerk` of the motor.
The prior art has attempted to eliminate or lessen this problem by using a rheostat to limit the starting current drawn from the power supply to a value less than that required by the motor in order to bring the motor up to speed by gradually reducing the resistance, but this method is inherently inefficient and converts the motor into an essentially variable-speed motor rather than an essentially constant-speed motor. Other means for controlling torque or speed are well known, but none use the teachings of the present invention.
This invention provides a clutch connected between the power output shaft and the rotor which will slip and permit the shaft to run at a lower speed than the rotor and thereby limit the power transmitted to the downstream components when the motor is starting up. A representative power output would be 150% of rated torque. Since the maximum torque that may be transmitted to downstream components is thereby limited, the components need not be designed to withstand a high load, but can be decreased in size and ruggedness, thus considerably reducing the cost of such components.
It is an object of the present invention to provide a clutch connecting the rotor and the output shaft of a polyphase motor to prevent the output shaft from transmitting more than a preselected torque to downstream components.
Another object of this invention is a clutch connected between the rotor and output shaft of a polyphase motor which permits the output shaft to slip and run at a lower speed than the rotor when the output shaft produces a torque higher than a preselected value.
A further object of this invention is the use of a friction clutch at each end of the rotor of a polyphase motor which prevents excessive power being applied to the load upon start up of the motor.
A still further object of his invention is an adjustable clutch between the rotor and output shaft of a motor, and an adjustable spring means for varying the torque conditions at which slippage occurs.